Airborne Molecular Contamination: Formation, Impact, Measurement and Removal of Nitrous Acid (HNO2) Jürgen M
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MICROCONTAMINATION CONTROL | APPLICATION NOTE Airborne Molecular Contamination: Formation, Impact, Measurement and Removal of Nitrous Acid (HNO2) Jürgen M. Lobert, Reena Srivastava, and Frank Belanger – AMC Business Unit, Entegris, Inc. Franklin, MA USA | [email protected] ABSTRACT Acidic gases have been a concern since the inception of 193 nm — technology. Initially focused on strong acids that cause corrosion Airborne molecular contamination (AMC) is a significant contributor on equipment or hazing of optics through the formation of salts to the loss of yield in semiconductor processes.1-4 The impact of (reactions with ammonia, for example),2-4 more recent concerns weak acids has only been considered for process technologies of are about the weaker acidic compounds, such as formic and 5-8 acetic acids, which can cause process degradation by affecting 22 nm and below. One such weak acid is nitrous acid (HNO2 or HONO), which has no demonstrated direct impact on processes or resist8 and other coatings. equipment, but has nevertheless been a target for removal by AMC Although no strict definition exists, weak acids are considered filtration. HNO is commonly formed on all surfaces in all environ- 2 to be those with a pKa of 3.2 or higher (acid strength, lower ments from NO gas, one of the main oxides of nitrogen formed 2 number indicates stronger acid), which includes formic, acetic from combustion processes and ambient air photochemistry. and other organic acids, but also nitrous acid (HNO , pKa = 3.3) This study investigated the behavior of the NO /HNO system 2 X X and hydrofluoric acid (HF, pKa = 3.2). In contrast to its strongly around typical AMC filter adsorbents. We find that NO gas passes acidic counterpart nitric acid (HNO , pKa = -2), however, HNO is through AMC filters unchanged, whereas NO is converted mostly 3 2 2 not a stable compound and its formation can easily be reversed to NO, but also to HNO at the low ppb level, increasing AMC load 2 into its components. As a result, HNO is rarely considered in downstream of filters. Various adsorbents can capture HNO , but 2 2 atmospheric chemistry cycles and very few dedicated studies filter lifetimes are short due to the release of the volatile compound exist.9, 10 In contrast to a variety of other nitrogen compounds over time. The recommendation is to critically evaluate the impact like NO , HNO is also not usually found in significant amounts of HNO on processes and equipment and adjust AMC filtration x 2 2 in the combustion process of nitrogen containing fuels.11 needs accordingly. NOMENCLATURE INTRODUCTION — — A common misnomer used in the semiconductor industry is Controlling Airborne Molecular Contamination (AMC) has become “NO ”. The atmospheric science community has referred to the an indispensable part for high yield operations in semiconductor X mix of NO and NO gases as NO for decades, and the authors processing. What started decades ago with concerns about a few 2 X propose to continue the exclusive use of that acronym for this base compounds, such as ammonia1, expanded to controlling most purpose. The scientific community also uses the acronym NO measurable gas-phase contaminants classified as acids, bases, y to refer to the mix of other reactive nitrogen oxides, such as and organic compounds, plus some unclassified compounds like PAN (peroxy acyl nitrate), N O , N O etc., all commonly found hydrogen sulfide, ozone etc., down to parts per trillion (ppt, 10-12 2 3 2 5 in the atmosphere. mole fraction) levels in order to achieve semiconductor features below 22 nm, and to prevent critical dimensions from being degraded, coatings from delaminating, or metal surfaces from corroding.5-8 However, there is no such acronym in atmospheric B. Formation of HNO2 on surfaces chemistry to describe the sum of acidic species HNO2 Typical concentrations of HNO2 that are found by this and HNO3, which is what the semiconductor industry laboratory in various semiconductor environments, incorrectly calls “NOX”. Common use also suppresses the minus signs in ionic forms, leading to further such as air handlers, cleanrooms, scanner, and track -9 – tools, are a few parts per billion (ppb, 10 mole fraction), confusion between HNO2 in water (NO2 , nitrite) and mostly much below 10. However, much of the found the gas NO2. Here are the most common forms of HNO can be a result of an artifact described in the oxidized nitrogen gases: 2 next section. NO: nitrogen oxide; NO2: nitrogen dioxide What complicates the evaluation of the impact of HNO2 NOX: Σ NO+NO2 is its instability. It is mainly formed from NO and NO2, two reactive gases from combustion processes (car NO : NO, NO , PAN, reactive oxides Y Σ 2 exhausts, heating systems, industrial emissions, etc.). (“odd nitrogen”) The primary formation of HNO2 on surfaces mentioned above explains why high surface adsorbents, such as HNO2: nitrous acid, a weak acid activated carbons, may be effective in triggering the without known impact conversion of NO2 to HNO2. Such highly porous – Nitrite: NO2 , the ionic form of HNO2 surfaces are found in AMC chemical filters, and they in water can accelerate that formation to be much faster than through these more common, but slow, gas-phase HNO3: nitric acid, a strongly corrosive acid reactions: – Nitrate: NO3 , the ionic form of HNO3 NO + NO2 HNO2 (1) in water 2NO2 +H2O HNO2 + HNO3 (2) HNOX: Σ HNO2+HNO3 (suggested acronym) Any formation of HNO2, however, is easily reversible – Virtual NOX : HNO2 artifact formed in and yields the originating gas NO2 or NO and •OH 15 water solutions radicals. High surface carbon filters, however, cannot be avoided for most applications, because they are If any acronym is needed, instead of explicitly stating needed to remove organic AMC from the airstreams, the individual acids, the authors propose to adopt HNO x something that is not easily or cheaply done with non- to indicate the acidic character and avoid confusion carbon based adsorbents. This formation pathway is with NO . X currently the main concern in the industry, as it makes AMC filters appear to be “outgassing” HNO2 when SOURCES AND REACTIONS FOR HNO2 applied to environments with significant NOX challenge. A. Atmospheric HNO2 C. Formation of HNO2 in air samplers Atmospheric HNO2 forms primarily through hetero- geneous reactions on surfaces (e.g., particles), as Another issue that exacerbates evaluating the impact witnessed by highest concentrations found during of HNO2 is its measurement. HNO2 is unstable in air, nighttime, when available airborne particle surfaces but also forms and decomposes in aqueous solution. are highest.11, 13 The compound is also destroyed on The compound is not available or stable in pressurized surfaces (equilibrium) and through photolysis, reactions gas tanks. induced by light, explaining its distinct diurnal cycle The industry and associated laboratories most com- when local sources are absent.9 In the gas phase, HNO 2 monly use impingers /bubblers, devices that contain is an unstable, short-lived intermediate between the deionized water through which the air of interest is stable forms of NO and HNO . Concentrations of X 3 drawn with the use of a pump to dissolve all soluble actual gas-phase HNO in the atmosphere are usually 2 contaminants (usually acids and bases) in that water at the ppt level and up to 1 ppb, hence, not of concern for later analysis by ion chromatography 14 (Figure 1). to semiconductor processing at this time. 2 range, and was found to be as high as 1000 ppb in the vicinity of combustion sources, the amount of HNO2 measured in impinger samples can quickly approach several ppb, in turn triggering alarms for the level of acidic AMC. The cited application note mentions a potential workaround for the artifact, but it involves duplicate equipment and is typically not done by laboratories (Figure 2). Taken together, atmospheric HNO2 is negligible in most cases. The amounts of HNO2 produced in impinger/ bubbler sampling can significantly contribute to re- ported values, but the HNO2 formed by high surface adsorbents may be the biggest contributor to the measured HNO2 (or dissolved nitrite) concentration in semiconductor environments. IMPACT OF HNO2 ON PROCESS AND EQUIPMENT — Based on a lack of understanding the chemistry and instability of the compound, the main concern about Figure 1. Typical deployment of a wet impinger/bubbler for the HNO in semiconductor environments appears to be determination of acids and bases. 2 perceptual, as that any presence of nominally “acidic” The authors reported before about the formation compounds is generally undesired. However, there 15 artifact of HNO2 in water impingers/bubblers. The are many organic acids that should trigger similar – effect was named Virtual NOx (note the minus sign concerns, but are typically ignored, such as lactic indicating the ionic form), because the resulting HNO2 acid, which often is detected in semiconductor that forms in the aqueous solution is artificial, creating environment air samples. a ‘virtual’ and inflated signal for actual nitrite concen- tration. This effect is based on the conversion of NO Whereas strong acids can significantly impact process – steps and equipment at the ppb level, however, there and mostly NO2 to HNO2 (as nitrite, NO2 ) in water, is no published or direct impact of HNO on any semi- along the lines of Eq. 1, which happens faster in water 2 than in the gas-phase. conductor process step or equipment and in many cases, its existence at single digit ppb levels in semi- conductor samples has been largely ignored for years by equipment and chipmakers. The only issue the authors are aware of is that HNO2 may possibly facilitate corrosion of bare metals by hydrogen sulfide (H2S), but it is the H2S causing the corrosion, not the HNO2, the latter only attacking a protective coating to let the H2S penetrate to the 17 metal.